Corruption of
~/Library/Mail/Envelope Index is known to cause this bogus error message in Mail even when there is enough disk space available (but you must make sure that there is enough space available first).
Verify/repair the startup disk (not just permissions), as described here:
The Repair functions of Disk Utility: what's it all about?
After having fixed
all filesystem issues, if any, and making sure that there's enough space available on the startup disk (a few GB, plus the space needed to make a backup copy of the Mail folder), try this:
1. Quit Mail if it's running.
2. In the Finder, go to
~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong -- e.g. by dragging it to the Desktop while holding the Option (Alt) key down.
3. Locate
Envelope Index and move it to the Trash. If you see an
Envelope Index-journal file there, delete it as well.
4. Open Mail. It will tell you that your mail needs to be "imported". Click
Continue and Mail will proceed to re-create
Envelope Index -- Mail says it's "importing", but it just re-creates the index if the mailboxes are already in Mail 2.x format.
Note: For those not familiarized with the ~/ notation, it refers to the user's home folder, i.e. ~/Library is the Library folder within the user's home folder.
